This invention relates to environmental monitoring systems and in particular to apparatus which samples the workability and reliability of environmental detector units coupled in such systems.
Environmental monitoring systems are useful to monitor environmental abnormalities such as, fire, smoke, water leakage, gas leakage and the like at various locations in buildings, building complexes and/or other properties.
Such monitoring systems generally include a number of detector units dispersed in various locations throughout the property, each designed with a sensor to sense an environmental parameter (e.g., temperature, smoke, gas, water, etc.) for abnormality and to generate a signal indicative of such abnormality. A computer is arranged to supervise the detector units for abnormality data collection operations and control operations. Among the control operations is a test for workability of a sensor, say a smoke sensor.
One known technique for testing such sensors involves maintenance personnel walking through the property and deliberately activating the sensors, for example, injecting smoke into smoke sensors to simulate the abnormality condition. The sensor output signal is captured at the detector location and collected by the central computer for evaluation of sensor workability. A disadvantage of this technique is the need for walk through personnel and their need for communication to an operator at the central computer. Because of the labor cost and the time required to conduct such a walk through test, the testing operation is conducted rather infrequently.
Another known technique for sensor workability testing is described in U.S. Pat. No. 4,518,952 which discloses an alarm system in which the sensors are tested for workability from a remote location, thereby obviating the need for walk through maintenance personnel. In this system the test involves a computer (located remotely of the sensors) which sends to a terminal unit having a sensor to be tested a test instruction signal. The terminal unit responds by generating a predetermined test voltage that is applied to the sensor. The sensor responds with an analog output voltage that is the sum of the test voltage and the ambient voltage of the sensed parameter. This analog output voltage is then converted in an analog to digital converter to a digital signal that is sent to the computer for evaluation.
The system of the above-mentioned patent does not disclose any means for dealing with tolerance problems such as (a) variations in the ambient signal conditions (e.g., a room that is sometimes vacant and at other times occupied with smokers, or simply changes in humidity and/or dirt content in the air) and/or (b) drift of the analog to digital converter due to aging, component selection, etc. Moreover, there is no disclosure of how often or on what basis the test is to be conducted relative to data collection operations performed in the system.
One solution to the tolerance problems is to set the abnormality threshold (and the predetermined test voltage) so as to yield the maximum digital output value of the analog to digital converter for the abnormality condition. Although this solution may be suitable for determining workability of the sensor, degradation or drift of the analog to digital converter could very well go undetected.